Understanding the T Cells That Cause GVHD: An Ever-replenishing Pool, or Tissue-specific Reservoirs?

Acute graft-versus-host disease (GVHD) is a major problem for patients undergoing allogeneic hematopoietic cell transplantation. Donor T cells — critical for infectious immunity, engraftment, and graft-versus-leukemia effects — are, problematically, also the drivers of acute GVHD. Strategies to prevent GVHD largely focus on systemic T-cell depletion (e.g., with post-transplant cyclophosphamide) or suppression (e.g., with calcineurin inhibitors like tacrolimus). Treatment strategies also target T cells and include high-dose steroids and the JAK/STAT inhibitor ruxolitinib. Despite the availability of these systemic prevention and treatment strategies, GVHD clinically manifests as a tissue-specific syndrome, predominantly affecting the skin, gastrointestinal tract, and liver. While we understand many things about GVHD biology, there are still many open questions.

Dr. Sacirbegovic and colleagues have used carefully designed combinations of monoclonal transgenic T cells in mouse models of transplantation to interrogate the distribution of host-reactive T-cell clones from the donor graft into recipient tissues. Mouse models are required to address these kinds of questions because real-time localized tissue assessment in transplant patients is not possible. By generating a set of specific T cells as reagents and infusing these uniquely identifiable single cells as starting clones, the investigators were able to identify specific T-cell clones in mouse tissues and track their maintenance during the weeks following transplant.

Early during the course of GVHD, potentially alloreactive T-cell clones were found in a broad range of tissue types. The experimental design specifically facilitated the tracking of individual clones over time, allowing the authors to explore an important unanswered question in GVHD biology: Does the composition of alloreactive T cells in different tissues become more similar over time (suggesting a common, circulating alloreactive T-cell pool that replenishes the tissues), or do the tissues diverge with regard to clonal composition (suggesting that local tissue maintenance of T-cell activity is critical for disease)?

Intriguingly, the experimental data paired with elegant mathematical modeling demonstrate that alloreactive T-cell clones expand locally within target tissues and do not seem to be exclusively replenished from the periphery (e.g., from a central priming site like the mesenteric lymph node). This finding prompts another question: How can these chronically antigen-exposed, exhausted T cells living within damaged tissue continue to be pathogenic?

To address this question, the authors purified antigen-specific unique T-cell clones and performed single-cell RNA sequencing to identify the breadth of transcriptomic features of these clonal T cells that persist within the skin, small and large intestines, and spleen. They identified an “exhausted progenitor” subset of cells that express the marker TCF-7 and went on to perform several functional transplant experiments to assess their capacity to act as parents to a pool of T cells capable of maintaining GVHD.